Injection molding can be used to form parts that have complex shapes and features. It is known to use injection molding to encapsulate other parts or components, such as stamped metal conductors in the manufacture of electrical parts. A simple example is the common formation of a plastic plug head on the end of an electrical cord. Electrically conductive metal blades for the plug are electrically connected to wires. The blades are then positioned in a mold where plastic is injected to encapsulate an inner end portion of the blades as well as the blade-to-wire connection. The portions of the blades not to be encapsulated in plastic, the portions that are to remain exposed for electrical connection to another cord or to an electrical outlet, are inserted into narrow slots in the mold steel, the slots leaving insufficient space around the blades for the injected plastic to flow therein. More complex processes are used for more complex products, but the general process remains the same, with parts of components encapsulated in the injection molded plastic and other parts not encapsulated.
When molding plastic parts, it is desirable to maintain a specific cycle time, i.e. the time required to initiate the process for the part, mold the part and remove the part from the mold to start the next cycle. Desirably, the cycle time is sufficiently short to keep the plastic resin from degrading due to exposure to excessive heat in the molding machine injection unit, and to make the part in the allotted time.
It is known in the plastic injection molding industry that encapsulating a metal stamping that possesses multiple circuits creates a challenge if the multiple circuits must be handled and loaded into the mold by a human molding operator. It is desirable to load multiple circuits in a timely manner to maintain a specific cycle time. The more circuit components that must be loaded into a mold, the more time it takes to prepare the component parts before the actual step of injecting the plastic can be initiated. The same is true for any molding process in which multiple individual component parts have to be positioned in the mold before plastic injection can be initiated. Accordingly, it is desirable to shorten the time it takes for a mold operator to load metal stampings with multiple circuits, or other individual component parts, so that mold use efficiency is optimized.
It is known to attach multiple circuits together as a one-piece stamping assembly that a molding operator can load into a mold as a single piece. A known way to attach multiple circuits is to connect them via a “tie-bar”, which is a portion of the metal stamping which is left between two or more circuits during the stamping process. Several tie-bars can be used to hold multiple circuits together, effectively creating a one-piece stamping for handling purposes. After the plastic part is molded, the tie-bar or tie-bars must be removed, often using a trim die that cuts or stamps the tie-bar from its position between the circuits, thereby effectively separating the circuits both physically and electrically. The use of a trim die in this way creates an open hole through the part when the die is pushed through the part to remove the tie-bar connection. A trimming operation adds additional cost to the part due to the additional handling after molding, and also requires the part to be designed in such a way as to have access to any tie-bars from both sides of the molded part for the trimming operation. In some part designs, it is difficult, if not impossible, to have the space required for tie-bars; or it may not be acceptable or desirable to have an open hole through the finished part. Some plastic parts must be sealed from one side of the part to the other side of the part, and having an open hole through the part must be avoided.